Alignment method for optimizing extinction ratios of coated polarizing beam splitters

ABSTRACT

An optical element such as a beam splitter for interferometer separates a heterodyne beam from the laser into separate beams having different the frequencies and orthogonal polarizations. Optical fibers can conduct the separate beams to a beam combiner for interferometer optics. The PBS and/or the beam combiner can use a coating to reflect one linear polarization and transmit an orthogonal linear polarization. To improve extinction ratios in the PBS or the beam combiner, a yaw angle for an input beam is non-zero and corresponds to a peak in the extinction ratio of a reflected beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent document is a divisional and claims benefit of theearlier filing date of U.S. patent application Ser. No. 09/933,622,filed Aug. 20, 2001, which is hereby incorporated by reference in itsentirety.

BACKGROUND

[0002] Two standard types of PBSs include polarizing beams splitterscontaining birefringent materials (often referred to herein asbirefringent PBSs) and polarizing beam splitters using thin-filmcoatings (often referred to herein as coated PBSs). Generally, coatedPBSs are much less expensive and more durable than birefringent PBSs.Additionally, all the materials in a coated PBS can be “factory-made” toa high degree of purity, but calcite, which is the most commonly usedbirefringent material in birefringent PBSs, currently cannot beartificially manufactured and supplies of calcite found in nature aredwindling.

[0003]FIG. 1 shows a cross-section of a coated PBS 100 including athin-film coating 120 between two pieces of isotropic glass 110 and 130.Glass pieces 110 and 130 are prisms having cross-sections that areisosceles right triangles. Thin-film coating 120 is on one piece ofglass (e.g., prism 110) and a glue layer that binds the pieces is on theother piece of glass (e.g., prism 130).

[0004] Thin-film coating 120 generally includes multiple layers of twoor more materials having different indices of refraction. The layermaterials, the number of layers, and the thickness of each layer areselected to transmit light having a first linear polarization P andreflect light having a second linear polarization S.

[0005] As illustrated in FIG. 1, an input beam IN containing componentswith both polarizations P and S is incident normal to the surface ofglass 110 and at 45° to the normal of thin-film coating 120. Ifthin-film coating 120 is properly constructed, a beam TOut, which istransmitted through thin-film coating 120, predominantly contains lighthaving the first polarization P, and a beam ROut, which is reflectedfrom thin-film coating 120, predominantly contains light having thesecond polarization S. Generally, each output beam TOut and ROut willhave polarization components with both polarizations P and S. The ratioof the intensity of the predominant polarization component P or S to theother polarization component S or P is commonly referred to as theextinction ratio.

[0006] Coated PBSs generally do not perform as well as birefringentPBSs. Many birefringent PBSs can provide extinction ratios greater than10,000:1 for both transmitted and reflected beams. Most commerciallyavailable coated PBSs produce one “good” polarization (usually thetransmitted beam) having an extinction ratio of about 500:1 or betterand a not-as-good polarization having an extinction ratio of less thanabout 200:1. Most optics catalogs and manufacturers specify theextinction ratio of the transmitted beam, and the extinction ratio forthe reflected beam (if indicated) is relatively poor.

[0007] Many applications, including interferometers and some fiberopticgyroscopes, require two beams with polarizations that are highly linearand orthogonal, for example, output beams from a PBS having extinctionratios greater than about 1,000:1. These applications have commonly usedbirefringent beam splitters or added “clean-up” polarizers to theoutputs of a coated PBSs. Adding polarizers to the output of a coatedPBS increases system cost and complexity and also requires additionalalignment processes. Development of better coatings that provide higherextinction levels may be able to achieve high extinction ratios incoated PBSs, but methods for improving the extinction ratios of coatedPBSs without increasing system cost and complexity are sought.

SUMMARY

[0008] In accordance with an aspect of the invention, an alignmentprocedure for a coated polarizing beam splitter (PBS) adjusts the yawangle so that the input beam is at a non-zero angle to the normal of theinput surface of the coated PBS. The yaw angle, which is typically lessthan 10° from the normal, generally has little effect on the extinctionratio of the transmitted beam, but the extinction ratio for thereflected beam varies more rapidly and commonly has a peak within theyaw angle alignment range. The alignment procedure finds the yaw angleproviding the best extinction ratio for the reflected beam.

[0009] The yaw angle alignment procedure can also improve theperformance of a beam combiner by improving rejection of unwantedpolarizations from input beams so that a combined beam has linearorthogonal components.

[0010] One specific embodiment of the invention is an interferometerthat includes a laser, a beam splitter, and interferometer optics. Thelaser can use Zeeman splitting to produce a heterodyne beam containingtwo frequency components having different frequencies and circularpolarizations, and an optical element such as a quarter-wave plate canconvert circular polarizations of two frequency components intoorthogonal linear polarizations. The coated beam splitter uses thedifference in linear polarizations to split the heterodyne beam intoseparate beams having different frequencies. To improve performance ofthe coated PBS, the heterodyne beam has a non-zero incidence angle withthe PBS. In particular, the non-zero incidence angle corresponds to apeak in the extinction ratio of a beam reflected in the PBS. With thealignment-enhanced performance, the separate beams have very linear andorthogonal polarizations.

[0011] Optical fibers carry the separate beams to the interferometeroptics. A beam combiner can recombine the separate beams in to aheterodyne beam used by the interferometer optics to generatemeasurement and reference beams. The beam combiner can be a coated PBSthat is oriented to receive the separate beams at non-zero incidenceangles that correspond to a peak in the extinction ratio of a reflectedbeam in the coated PBS.

[0012] Another embodiment of the invention is an optical element such asa coated PBS or a beam combiner that includes a beam splitter coatingbetween first and second pieces of glass. The optical element isoriented to receive an input beam at a non-zero incident angle with anormal to a surface of the first piece of glass. Ideally, the non-zeroincident angle corresponds to a peak extinction ratio for a beamreflected from the beam splitter coating.

[0013] Yet another embodiment of the invention is a method for aligningan optical element such as a coated PBS or a beam combiner containing aPBS coating. The method includes: directing an input beam along a firstaxis into the optical element; rotating the optical element to change ayaw angle of incidence of the input beam; observing a reflected beamresulting from reflection of a portion of the input beam in the PBScoating; and mounting-the element at the yaw angle that the observingindicates provides a best extinction ratio for the reflected beam.Observing the reflected beam can include measuring light intensitypassing through a polarizer that is in a path of the reflected beam andhas a polarization axis orthogonal to a desired polarization of thereflected beam.

[0014] In addition to adjusting the yaw angle, the roll and/or the pitchangles can be similarly adjusted. For example, adjusting the roll anglecan be performed by: rotating the optical element to change the rollangle of incidence of the input beam; observing a transmitted beamresulting from a portion of the input beam passing through the PBScoating; and mounting the optical element at the roll angle that theobserving of the transmitted beam indicates minimizes presence of afirst frequency in the transmitted beam. Adjusting the pitch angle caninclude rotating the optical element to thereby change the pitch angleof incidence of the input beam and adjust a path of a beam resultingfrom a portion of the input beam reflected by or transmitted through thePBS coating.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a ray trace diagram for a known coated polarizing beamsplitter.

[0016]FIG. 2 is a block diagram of a two-frequency interferometer inaccordance with an embodiment of the invention.

[0017]FIG. 3 illustrates the alignment of a coated polarizing beamsplitter in accordance with an embodiment of the invention.

[0018]FIG. 4 shows plots illustrating the dependence of the extinctionratios on yaw angle for a typical coated polarizing beam splitter.

[0019]FIG. 5 is a flow diagram of a process for aligning a coated beamsplitter in accordance with an embodiment of the invention.

[0020]FIG. 6 illustrates the alignment of beam combiner in accordancewith an embodiment of the invention.

[0021]FIG. 7 is a flow diagram of a process for aligning a beam combinerin accordance with an embodiment of the invention.

[0022] Use of the same reference symbols in different figures indicatessimilar or identical items.

DETAILED DESCRIPTION

[0023] In accordance with an aspect of the invention, an alignmentmethod optimizes the extinction ratio for the reflected beam of a coatedpolarizing beam splitter (PBS) by adjusting the incident direction of aninput beam. The change in incident direction can significantly improvethe extinction ratio for the reflected beam without significantlycompromising the extinction ratio of the transmitted beam. The alignmentmethod can improve the reflected extinction ratio enough to permit useof a coated PBS in a demanding application such as in a two-frequencyinterferometer.

[0024]FIG. 2 is a block diagram of an interferometer system 200 inaccordance with an embodiment of the present invention. Interferometersystem 200 includes a laser 210, a quarter-wave plate 215, a coatedpolarizing beam splitter 220, acousto-optical modulators (AOMs) 230 and235, optical fibers 250 and 255, adjusting optics 260, a beam combiner270, a beam splitter 275, and interferometer optics 290.

[0025] Laser 210 and quarter-wave plate 215 act as a source of aheterodyne beam having two distinct frequency components with orthogonallinear polarizations. An exemplary embodiment of laser 210 is acommercially available He—Ne laser such as a Model 5517D available formAgilent Technologies, Inc., which uses Zeeman splitting to generate thetwo frequency components in the same laser cavity. Zeeman splitting inthis manner can generate a heterodyne beam have frequency componentswith frequencies f1′ and f2′ and a frequency difference of about 2 MHz.The two frequency components have opposite circular polarizations, andquarter-wave plate 215 changes the polarizations of the frequencycomponents so that the frequency components have orthogonal linearpolarizations.

[0026] Coated PBS 220 separates the two frequency components. Coated PBS220 can be commercially available high quality beams splitter availableform Optisigma, Inc. Generally, coated PBS 220 is designed to maximizethe extinction ratio of the transmitted beam. To improve the extinctionratio of the reflected beam, polarizing beam splitter 220 is rotated toa yaw angle that provides the best results and the cleanest separationof the frequency/polarization components. Accordingly, the input beaminput generally is not normal to the entrance surfaces of polarizingbeam splitter 220.

[0027] In the illustrated embodiment, the lower frequency component hasa polarization that coated PBS 220 transmits to AOM 230, and the higherfrequency component has the polarization that coated PBS 220 reflectstoward AOM 235. AOMs 230 and 235 operate at different frequencies (e.g.,80 MHz and 86 MHz) and change the frequencies of the two beams tofurther separate the frequencies of the two beams. The beams output fromAOMs 230 and 235 have respective frequencies f1=f1′+80 MHz and f2=f2′+86MHz that are about 8 MHz apart. The wider frequency separation allowsinterferometer system 200 to accurately measure faster moving objects.

[0028] The embodiment of FIG. 2 uses two AOMs 230 and 235 operating atcomparable frequencies (e.g., 80 and 86 MHz). This has the advantage ofmaking the optical paths and influences on the two separate beams morecomparable. Additionally, neither AOM needs to be operated at a lowfrequency (e.g., 6 MHz) to increase the frequency difference by arelatively small amount. However, an alternative embodiment of theinvention could employ a single AOM to shift the frequency of one of thebeams and thereby increase the frequency difference.

[0029] Lenses 240 and 245 focus the separate beams into separatepolarization preserving optical fibers 250 and 255, respectively. In anexemplary embodiment of the invention, polarization-preserving opticalfibers 250 and 255 are commercially available optical fibers such asavailable from Corning, Inc. or Fujikura America, Inc. In someapplications, optical fibers 250 and 255 may traverse bulkheads or otherfixtures. Polarization-preserving fibers 250 and 255 deliver theseparate beams to adjustment optics 260 that direct the two beams into abeam combiner 270.

[0030] The use of optical fibers 250 and 255 allows the laser 210 andAOMs 230 and 235 to be mounted away from interferometer optics 290.Accordingly, heat generated in laser 210 and AOMs 230 and 235 does notdisturb the thermal environment of interferometer optics 290.Additionally, laser 210 and AOMs 230 and 235 do not need to have fixedpositions relative to interferometer optics 290, which may providesignificant advantages in applications having limited available spacenear the object being measured.

[0031] Adjustment optics 260 precisely align beams INR and INT fromoptical fibers 250 and 255 for combination in beam combiner 270 to forma collinear beam COut. A variety of optical/mechanical systems can beemployed for adjustment optics 260 and generally have configurationsthat depend on the available space and the maximum curvature of opticalfibers 250 and 255 that will sufficiently preserve the intensity andpolarization of the beams being carried. A co-filed U.S. patentapplication entitled “Direct Combination of Fiber Optic Light Beams”,Attorney Docket No. 10010323, which is hereby incorporated by referencein its entirety, further describes adjustment optics that position beamsfor combination.

[0032] Beam combiner 270 can be a coated PBS that is used in reverse. Inparticular, input beam INT, which predominantly has the polarizationthat the coated PBS transmits, is input along the direction of a desiredcombined output beam COut. Input beam INR, which predominantly has thepolarization that the coated PBS reflects, is input along the directionperpendicular to the desired combined output beam COut. Beams INT andINR generally include polarization components other than those desiredin combined output beam COut. These unwanted components can arise fromthe finite extinction ratios of coated PBS 220 or from AOMs 230 and 235and fittings or other structures in associated optical fibers 250 and255 changing the polarization. In accordance with an aspect of theinvention, the coated PBS forming beam combiner 270 can be rotatedslightly to a yaw angle that maximizes the extinction ratio of thereflected beam.

[0033] In an alternative embodiment, beam combiner 270 contains abirefringent material such as calcite. A co-filed U.S. patentapplication entitled “Birefringent Beam Combiners For Polarized Beams InInterferometers”, Att. Docket No. 10010511 describes beam combinerscontaining birefringent materials and is hereby incorporated byreference in its entirety.

[0034] Combined beam COut is input to a beam splitter 275 that reflectsa portion of beam COut for analysis systems 280. Analysis system 280uses the two frequency components of light reflected in beam splitter275 as first and second references beams. The remaining portion ofcombined beam COut enters interferometer optics 150.

[0035] In interferometer optics 290, a polarizing beam splitter 292reflects one of the polarizations (i.e., one frequency beam) to form athird reference beam directed toward a reference reflector 298 andtransmits the other linear polarization (i.e., the other frequency) as ameasurement beam toward an object being measured. In an alternativeversion of the interferometer optics, a polarizing beam splittertransmits the component that forms the measurement beam and reflects thecomponent that forms the reference beam.

[0036] Movement of the object causes a Doppler shift in the frequency ofthe measurement beam that analysis system 280 measured by combining themeasurement beam with the third reference beam to form a beat signal,having a frequency that is equal to the difference between thefrequencies of the third reference beam and the measurement beam afterreflection from the object. The frequency of this beat signal can becompared to the frequency of a beat signal generated from a combinationof the first and second reference beams to accurately determine theDoppler frequency shift. Analysis system 280 analyzes the Dopplerfrequency shift to determine the speed of and/or distance moved by theobject.

[0037] For accurate measurements, interferometer system 200 requiresthat the two frequency components of combined beam COut have orthogonallinear polarizations for clean separation of frequency components ininterferometer optics 290. Otherwise, both frequency components will bein the measurement beam and the reference beam introducing other beatfrequencies that make measurement of the Doppler shift more difficultand less accurate. Adjusting coated beam splitter 220 to maximize theextinction ratios provides cleaner separation two frequency/polarizationcomponents. The extinction ratios of beam combiner 270 also provideadditional filtering or rejection of the undesired frequencies.

[0038]FIG. 3 illustrates an embodiment of the invention where an inputbeam IN is incident on a coated polarizing beam splitter 220 at anon-zero incidence angle A. The non-zero yaw angle A causes refractionof incident beam IN, a transmitted output beam TOut, and a reflectedoutput beam ROut at the air-glass interfaces. In particular, thetransmitted output beam TOut is parallel to incident beam IN but notcollinear with incident beam IN because refraction shifts output beamTOut by a distance D relative to a straight-line extension of incidentbeam IN. Refraction causes a similar shift in the reflected beam. Insystem 200 of FIG. 2, elements such as AOMs 230 and 235 that aredownstream of beam coated splitter 220 are aligned to the positions ofoutput beams TOut and ROut after coated beam splitter 220 is aligned forbest performance.

[0039] Generally, the extinction ratio for transmitted beam TOut of acoated beam splitter is relatively insensitive to the yaw angle, but theextinction ratio for reflected beam ROut has peaks on the order of 0.1°wide. FIG. 4 illustrates example plots 410 and 420 of the transmittedand reflected extinction ratios, respectively of a typical coated beamsplitter. As shown in FIG. 4, plot 410 of the extinction ratio for thetransmitted beam is peaked about normal incidence but is insensitive toincidence angle over a range of a few degrees. In contrast, plot 420 ofthe extinction ratio for the reflected beam peaks at a non-zero incidentangle and has several local maxima and minima. For extinction ratiosbehaving as illustrated in FIG. 4, an optimal yaw angle of about −2°provides best performance.

[0040] The locations of the peaks in extinction ratio for the reflectedbeams can be theoretically or experimentally predicted if the optimalyaw angle is reasonably consistent from lot to lot of coated beamsplitters. However, the exact location of the best peak can depend onprocess variations in the manufacture of the beam splitter coating, andan alignment process is required for each coated PBS to locate theoptimal yaw angle.

[0041]FIG. 5 is a flow diagram illustrating an alignment process 500 forcoated beam splitter 220 in interferometer 200 of FIG. 2. Beforealignment process 500, laser 210 and quarter-wave plate 215 are set upto provide input beam 210, which is a heterodyne beam having twofrequencies f1′ and f2′.

[0042] Alignment process 500 begins in 510 by adjusting the pitch angleof coated beam splitter 220. The pitch angle is about a horizontal axisperpendicular to input beam IN, and adjusting the pitch angle adjuststhe plane of the output beams TOut and ROut. Pitch adjustment can beused to keep the output beams parallel to a base plate for the mountingstructure. The pitch angle generally has little or no effect on thefrequency purity of output beams.

[0043] Step 520 then adjusts the yaw angle, which is about a verticalaxis that is perpendicular to the input beam. Yaw angle adjustment step520 rotates coated PBS 220 to an orientation that maximizes theextinction ratio of the reflected beam. The optimal yaw angle can beidentified by positioning a linear polarizer with a polarization axisorthogonal to the desired polarization of the reflected beam. Coatedbeam splitter 220 is then rotated until a minimum is found in the lightintensity transmitted through the polarizer.

[0044] After the optimal yaw angle is found, step 530 adjusts the rollangle to assure that coated PBS 220 optimally separates the frequencycomponents of the heterodyne beam from laser 210. The roll angle, whichis about the axis of input beam IN, is adjusted to minimize onefrequency component in the transmitted beam TOut. In particular, alinear polarizer is positioned with a polarization direction at about45° to the desired polarization of the output beam TOut. (The 45°orientation of the polarizer can be established before coated PBS 220 isin place.) With coated PBS 220 in place, frequency variations at thebeat frequency f2′-f1′ are observed, while coated beam splitter 220 isrotated around the axis of input beam IN to an orientation thatminimizes the beat frequency component.

[0045] After the roll angle has been optimized, step 540 readjusts theyaw angle to maximize the extinction ratio of the reflected.Readjustment corrects for any changes to that roll angle adjustment(step 530) may have had on the extinction ratio of the reflected beam.Since the yaw angle is readjusted after roll angle adjustment, the firstyaw angle adjustment in step 520 can be a coarse adjustment, while theyaw angle adjustment in step 540 performs a finer adjustment. Whencoated PBS 220 has its optimal orientation, the extinction ratios forboth transmitted and reflected output beams TOut and ROut and the wavedistortion can be checked to confirm that coated beam splitter 220provides the required performance.

[0046] A mounting structure for a coated PBS of a type currentlyavailable commercially should provide an adjustment range of about ±75mrad and a resolution of about 0.5 mrad for the roll, pitch, and yawangles of the coated PBS. One mounting structure that achieves thedesired range and resolution of roll, pitch, and yaw angles uses asection of a sphere held in contact with a conical base. The coated PBSis mounted on the spherical section with a center of the sphere attarget point in the PBS coating. The contact of the spherical sectionwith the conical base allows rotation or adjustment of any of the roll,pitch, or yaw angles without translation of the coated PBS. When thecoated PBS is properly positioned, it can be fixed in place by gluingthe spherical section to the conical base. The mounting structure mayadditionally provide a translation adjustment of the PBS for fineadjustment of the horizontal position of the reflected beam. However,translation adjustment may not be necessary if the mounts and/or sizesof optical elements immediately downstream of the coated PBS can adjustto or accommodate changes in output beam positions resulting from thealignment of the coated PBS.

[0047] As noted above, the performance of a beam combiner can alsobenefit from yaw angle adjustment. FIG. 6 shows a beam combiner 270having input beams INR and INT at non-zero incident angles. Adjustmentoptics 260 controls the relative angle and separation between inputbeams INT and INR. Generally, input beams INR and INT are co-planar andabout perpendicular to each other. Adjustment optics 260 and themounting structure for beam combiner 270 can co-operatively adjust theroll, pitch, and yaw angles of input beams INR and INT and adjust thepoints at which input beams INR and INT are incident on beam combiner270.

[0048]FIG. 7 is a flow diagram of an alignment process 700 for beamcombiner 270. Alignment process 700 starts with adjusting the roll angleof beam combiner 270 so that the output polarization axes respectivelycorrespond to the polarization axes of the measurement and referencebeams in interferometer optics 290. Adjustment optics 260 can rotateinput beams INT and INR so that their polarizations match thepolarization axes of beam combiner 270.

[0049] Step 720 adjusts the yaw angle of input beam INR to maximize theextinction ratio of the reflected beam. To determine the optimal yawangle, input beam INT can be blocked so that output beam COut containsonly the reflection of input beam INR. The yaw angle is then adjusted tominimize the light intensity passing through a polarizer having apolarization axis perpendicular to the desired polarization of thereflected beam.

[0050] Step 730 uses the pitch angle of beam combiner 270 for adjustmentof the output beam COut. Step 740 then adjusts the yaw angle andincident point of input beam INT to make the transmitted portion ofcombined beam COut collinear with the reflected portion of combined beamCOut.

[0051] As described above, the yaw angle of coated beam splitter can beadjusted to maximize the extinction ratio of the reflected beam.Accordingly, a less-expensive coated beam splitter can be used in highperformance applications that previously had required expensivebirefringent polarizing beam splitters. The yaw angle adjustment is alsoapplicable to beam combiner to provide better performance.

[0052] One application of the coated beam splitters and combiners inaccordance with the invention is an interferometer, which uses the beamsplitter to separate frequency/polarization components of a heterodynebeam from a two-frequency laser. AOMs can then increase the frequencydifference between the separated beams before the separate beams aretransmitted via separate optical fibers to interferometer optics. At theinterferometer optics, a beam combiner can combine the two separatebeams into a heterodyne beam having two frequency components with highlylinear and orthogonal polarizations.

[0053] Although the invention has been described with reference toparticular embodiments, the description is only an example of theinvention's application and should not be taken as a limitation. Variousadaptations and combinations of features of the embodiments disclosedare within the scope of the invention as defined by the followingclaims.

What is claimed is:
 1. A method for aligning an optical elementcontaining a polarizing beam splitter coating, comprising: directing aninput beam along a first axis into the optical element; rotating theoptical element to change an angle of incidence of the input beam on thepolarizing beam splitter coating; observing an output beam from thepolarizing beam splitter coating; and mounting the optical element atthe angle that the observing indicates provides a best extinction ratiofor the output beam.
 2. The method of claim 1, wherein the angle changedby rotating the optical element is a yaw angle of the incidence of theinput beam.
 3. The method of claim 2, further comprising rotating theoptical element to change a pitch angle of incidence of the input beamand adjust a path of a beam resulting from a portion of the input beamreflected by the polarizing beam splitter coating.
 4. The method ofclaim 2, wherein the output beam comprises a reflected beam resultingfrom reflection of a portion of the input beam from the polarizing beamsplitter coating.
 5. The method of claim 3, further comprising: rotatingthe optical element to change a roll angle of incidence of the inputbeam; observing a transmitted beam resulting from a portion of the inputbeam passing through the polarizing beam splitter coating; and mountingthe optical element at the roll angle that the observing of thetransmitted beam indicates minimizes presence of a first frequency inthe transmitted beam.
 6. The method of claim 1, wherein the opticalelement comprises a polarizing beam splitter.
 7. The method of claim 1,wherein the optical element comprises a beam combiner.
 8. The method ofclaim 1, wherein observing the output beam comprises: placing in a pathof the output beam a polarizer having a polarization axis orthogonal toa desired polarization of the output beam; and measuring light intensitypassing through the polarizer.